1. Field of the Invention
The invention relates to an engine control system for a direct injection-spark ignition type of engine, and, in particular, to a direct injection-spark ignition engine control system which controls a fuel injection timing when the engine operates with an air-fuel ratio of .lambda.&gt;1.
2. Description of the Related Art
Engine control system of this type incorporate in an exhaust line a NOx adsorbing type of lean NOx conversion catalyst which, on one hand, adsorbs NOx in the exhaust gas while the air-fuel mixture is leaner than a stoichiometric mixture and, on the other hand, desorbs or releases the NOx into exhaust gas for catalyzing reduction of the NOx while the air-fuel mixture is richer than a stoichiometric mixture. As is known from, for example, International Patent Application WO93/07363, such an engine control system controls the engine to operate with an enriched mixture under accelerating conditions or full loading operating conditions and with a lean mixture under the remaining operating conditions, so as to improve fuel consumption.
An engine control system for a direct injection-spark ignition type of engine known from, for example, Japanese Unexamined Patent Publication 7-119507 controls the engine to cause stratified charge combustion in a lower engine loading zone and homogeneous charge combustion in a high engine loading zone. While the engine operates with lower speeds in the high loading zone, a given amount of fuel is delivered in two steps through early and late split injection in a intake stroke, so as to diffuse a first half of fuel sprayed through the early split injection in the combustion chamber before the end of a intake stroke and the second half of fuel in the combustion chamber with its volume increased. This prevents a generation of rich or dense mixture over the top of a piston in a subsequent compression stroke that generally occurs when a given amount of fuel is sprayed all at once through non-split injection, which is desirable to prevent generation of smoke.
Another engine control system for a direct injection-spark ignition engine cooperates with a fuel injector which is direct to face the top of a piston and energized to spray a small amount of fuel preparatorily at the beginning of a intake stroke when the engine causes knocking. The fuel partly sticks to the top wall of the piston on a side of an intake port and bounces off the piston wall toward the intake port to cool the piston head and the combustion chamber on the intake port side with the heat of vaporization of the fuel. Such an engine control system is known from, for example, Japanese Unexamined Patent Publication 7-217478.
Although the lean NOx conversion catalyst described in the International Patent Application WO93/07363 lowers the emission level of NOx even while a fuel mixture is lean, NOx conversion performance of the lean NOx conversion catalyst is inferior to that of a three-way catalyst in a condition in which a stoichiometric fuel mixture is burnt and is not always satisfactorily. In view of this circumstances, it is desirable to restrain formation of NOx during lean mixture combustion by making a fuel mixture more leaner. Further, although fuel consumption is improved even more as a fuel mixture is made leaner, if the fuel mixture is lean in excess, the engine encounters sharp aggravation of fuel combustion, which always results in a problem of lean limits that affects drivability.
Concerning this point, it is assumed that even the engine described in International Patent Application WO93/07363 which is operated with homogeneous charge combustion even when a fuel mixture is lean or the engine described in Japanese Unexamined Patent Publication 7-119507 which is operated with homogeneous charge combustion and stratified charge combustion changed from one to another according to operating conditions is operated with lean homogeneous charge combustion in a middle engine loading zone. Since the homogeneous charge combustion does not permit a high lean limit, if the air-fuel ratio is controlled to be under the lean limit, it is hard to improve fuel consumption and lower the emission level of NOx sufficiently.